Correct (in 11.3+, you will be able to add a series file to the
patches.addon.tar.bz2 tarball, saving you the need of editing the main
series.conf).

This compiles already quite well :-) However, I also want to change the
kernel .config (increase the max and runtime number of UARTS). What
would be the best way to do so? What I did is to write a diff to
config.tar.bz2 and put additional lines in kernel-source.spec:

Patch: increase_nr_of_uarts.patch (after the "Source300" line)

%patch (after the "%setup" line)

Are there any drawbacks with that or is this the recommended way?

Well, there is no recommended way in 11.2, unfortunatelly. In 11.3+,
there is a config.addon.tar.bz2 placeholder, where you can package e.g.
a config.addon/i386/pae file with the config options you want to change.
I should really document this in README.SUSE.

3. Version number
=================

I changed the "Release" line of kernel-source.spec to

Release: 0.2.1.mycompany.1

Does this sound reasonable or is there a recommendation for private
numbering?